The high density of charge-carrier traps adjacent the interface of an insulating layer and a silicon semiconductor device body plays a major role in determining the magnitude of the dark current in imaging devices, the low-current gain in bipolar devices, storage times in memories and noise in insulated-gate field-effect transistors, and results in charge transfer inefficiency which limits the application of silicon charge-coupled devices particularly of the surface channel type in memory, filter and image-sensing integrated circuits. These charge traps (also called `fast surface states`) appear to result from silicon atoms which are only trivalently bonded at the interface. The insulating layer is typically of thermally grown silicon dioxide or another compound of the semiconductor material, and similar trivalent bonding can occur in the insulating layer to produce traps deeper in the insulating layer.
It is conventional practice to anneal these surface states between a thermally grown silicon dioxide layer and a silicon body by heating the body with the layer to a temperature not exceeding about 500.degree. C. in an atmosphere of hydrogen, or a hydrogen-nitrogen mixture, or wet nitrogen. The hydrogen is thought to saturate the remaining dangling bond of trivalently bonded silicon atoms. In this manner the density N.sub.ss of the traps with thermally grown silicon dioxide layers on silicon devices can be reduced to between 10.sup.9 and 10.sup.10 cm.sup.-2.eV.sup.-1. However hydrogen does not diffuse readily through silicon nitride layers, and so a plasma and heating treatment has been tried with silicon nitride layers by Goascoz et al of the Laboratoire d'Electronique et de Technologie de l'Informatique, Laboratoire de Microelectronique Appliquee as described in Note Technique LETI/MEA No. 1356, Oct. 5, 1979, which was presented at ESSDERC, Munich 1979.
In this known annealing treatment for silicon nitride layers, a high frequency plasma system similar to a conventional r.f. sputtering system is used to produce atomic hydrogen species. The device sample is maintained in the glowing area of the hydrogen plasma on a heated mount. The device samples used were either MNS capacitors comprising a metal electrode on a larger area silicon nitride layer on a silicon substrate or MNOS memory capacitors comprising a metal electrode on a larger area silicon nitride layer on a very thin silicon dioxide layer on a silicon substrate. Reference samples which were not subjected to the plasma treatment had a surface state density N.sub.ss of 1.5.times.10.sup.12 cm.sup.-2.eV.sup.-1 for the MNS capacitor and 6.times.10.sup.11 cm.sup.-2.eV.sup.-1 for the MNOS capacitor. When the plasma treatment was effected N.sub.ss of both samples were reduced to about 10.sup.10 cm.sup.-2.eV.sup.-1 with the samples heated to 400.degree. C. and less than 10.sup.9 cm.sup.-2 eV.sup.-1 with samples heated to 500.degree. C.